This invention concerns springs and more particularly mechanical compression springs as are used to provide a return force such as in cam operated dies.
In cam operated tooling installed on dies in forming presses, a slide is operated by the press motion to drive a tool used to form a feature on a part.
One or more return springs are compressed as the tool is advanced by a camming action produced by the slide. When the press platens are moved apart, the compressed spring or springs act to retract the slide.
In metal forming, it sometimes takes considerable force to withdraw a piercing tool from the part, such that a high return spring force is required at the start of the return motion. At the same time, a substantial slide travel is required. Very stiff coil springs have limited available travel for a given space due to the thick cross section required to develop a high spring rate.
This required combination of long travel and high peak forces has led to the development and use of “nitrogen” springs in which compressed nitrogen is sealed in a chamber, and further compressed by a piston having a projecting rod subjected to a load, creating a fluid spring.
Nitrogen springs are capable of generating considerable forces while allowing substantial travel, and are widely used as return springs in die cam units.
However, nitrogen springs have several disadvantages, including their initial high cost and their need to be serviced regularly, and the fact that high cycle rates are not possible due to excessive heat build up.
These nitrogen springs must be precharged with nitrogen under high pressure so as to have an initial high spring rate.
This initial high spring rate creates a high shock force when the cam unit is impacted by the cam driver since there is a relatively high initial resistance to cam movement caused by the precharging of the nitrogen spring.
These disadvantages of nitrogen springs led to the development of a variable force spring assembly described in U.S. Pat. No. 5,390,903, assigned to the predecessor company of the assignee of the present application.
The design disclosed in U.S. Pat. No. 5,390,903 houses springs of different stiffness in telescoped casings which are arranged to successively compress the springs, which are nested together, allowing a large range of movement with low forces developed initially and very high forces developed at the end of the stroke. A stack of Belleville springs is used as the high rate spring.
The inclusion of machined casings and other components makes that device relatively high in cost to manufacture, and is not easily adapted to varying applications since the casings and other components must be reengineered for each application. Also, the Belleville springs used are not well suited for high cycling rates, as fatigue failure commonly occurs when Belleville springs are subjected to such service.
For this reason, there has heretofore not been any mechanical spring arrangement available which has become commercially successful as a replacement for nitrogen springs.
Thus there is a need for a mechanical spring combination having a large travel, with an initial low spring rate during an initial range of compression and developing a high spring force through second shorter range of compression of the spring combination.